Once In a Blue Moon

Just how rare are two full moons in one month?

By J. Alex Knoll

The full moon lights up the night Friday, the second full moon of the month, a Blue Moon.
The history of the phrase Blue Moon dates back several hundred years, but the meaning has evolved. As far back as the 16th century, it was an expression of absurdity. I’ll believe that when the moon is blue would have the same effect as saying I’ll believe that when hell freezes over.
Atmospheric conditions can affect the color of the moon. Volcanic eruptions, dust storms and forest fires have filled the skies with enough airborne particulates to turn the moon blue. An 1883 eruption of the Indonesian volcano Krakatoa caused green sunsets and a blue moon for almost two years. So over time, the phrase became an expression of rarity, as in once in a blue moon.
Perhaps it is this off-chance, ever-so-slight possibility that lent a dose of melancholy and longing, immortalized in Elvis’ ballad, Blue Moon. “You saw me standing alone, without a love of my own …” But just maybe that Blue Moon would deliver love and happiness. Then, in the 1980s, the meaning as a single month with two full moons went viral. In a 2012 column for Sky & Telescope Magazine, Philip Hiscock traces the Blue Moon phrase to a 1946 Sky & Telescope Magazine article that was then quoted in a 1980 radio broadcast of the NPR program Stardate. Read the story at http://tinyurl.com/qhm28h5.
By today’s definition, a Blue Moon isn’t all that rare. The moon travels through its phases from one full moon to the next over a period of 281⁄2 days. Whenever a full moon falls in the first few days of a month, it’s likely a second Blue Moon will follow at the end of the month. In fact, during leap year a blue moon can even fall in February. A Blue Moon happens on average once every two to three years. But every now and then you might have two Blue Moons within three months, when a typical 28-day February has no full moon; That would leave January with two full moons followed by March with two full moons itself. This will next occur in 2018.
Blue Moons are in fact as predictable as clockwork. Every 19 years, in what’s called a Metonic cycle, the solar calendar and the lunar calendar are in synch. The ancient Greeks used this as the basis for their calendar, which stood until 46BC with the advent of the Julian calendar. Based on the Metonic cycle, 19 years from this Friday — or 235 lunar months — a Blue Moon will again fall on July 31.
The next day, August 1, marks another calendar milestone — Midsummer, third of the four cross-quarter days midway between solstice and equinox. The actual midpoint of summer falls on August 7 this year. The day of midsummer, once a pagan holiday called Lughnasadh in honor of the waning, post-solstice sun god, was co-opted by the Church during the early spread of Christianity, becoming Lammas Day, the festival of the wheat harvest, celebrated August 1.
Look to the west just after sunset for Venus and Jupiter. They’re still only six degrees apart and the two brightest star-like objects in the heavens. But they set within 45 minutes of the sun, and soon they will be lost in its glare.
That leaves Saturn ruling the night sky. As bright as an average star, the ringed planet appears high in the south at sunset and doesn’t set until after midnight.